It's a nice info that it once failed in the past. But once we know that, we get over it and want the warning to disappear. With the exclude list, the plugin can be told to ignore this attribute "Temperature_Celsius":

But this could also be a bit dangerous. What if the drive has a new (live!) temperature alert? You'd certainly want to know about it. That's why, besides excluding a SMART attribute, it is also possible to exclude certain values in the "When_failed" column. In the following example, the "When_Failed" value "In_the_past" (as seen above) can be used in the exclude list: